The term “aggregates” is generally used to describe a broad category of coarse particulate material used in construction, including sand, gravel and crushed stone. Aggregates are the most mined materials in the world. Aggregates are typically washed and graded on a combination of vibrating screens and hydrocyclones to produce washed aggregate products having a predetermined grain size or range of grain size. The term “aggregate” is more typically used to describe particulate material having a grain size of over 6 mm while the term “sand” is typically used to describe particulate having a grain sand of between 0.075 mm and 6 mm and such terminology will be used to describe such particulate material hereinafter.
A typical vibrating screen comprises a frame, defined by a pair of substantially parallel side walls interconnected by transversely extending bridging members, upon which is mounted one or more polyurethane decks having small openings or slots for water and undersize particles to pass through. The frame is typically mounted on a base via resilient linkages and the frame, and thus the deck or decks are typically vibrated by means of a pair of counter rotating rotors defining eccentric masses, driven by one or more drive motors, to impart circular or reciprocating vibrating motion to the deck(s). Such screens can be used for grading and/or dewatering particulate material, oversize material passing over the deck(s) of the screen to be collected from a downstream end of the screen while water and undersize material is collected in a sump of the screen for subsequent processing.
A hydrocyclone is a device used to separate particles in a liquid suspension based on the ratio of their centripetal force to fluid resistance. This ratio is high for coarse particles and low for fine particles. A hydrocyclone typically comprises a cylindrical section having an inlet for supplying a feed slurry into the hydrocyclone tangentially, and a conical base. Outlets are provided at upper and lower ends of the hydrocyclone. Underflow, containing the coarser fraction, passes out of the lower outlet while overflow, containing the finer fraction and most of the water, passes out of the outlet at the upper end of the hydrocyclone. Thus, as well as removing fine contaminants, a hydrocyclone also serves to reduce the water content of a sand/water slurry.
Hydrocyclones are often used in combination with vibratory screens for washing and dewatering sand, a slurry of sand and water being cleaned and dewatered in one or more hydrocyclones before the sand, contained in the underflow from the hydrocyclones, is delivered onto a deck of a vibratory screen for grading and/or further dewatering.
Most sand and aggregate washing and grading plants are very large, including different stages comprising multiple grading and dewatering screens and hydrocyclones, and typically require a large volume of water to fluidise the material in each stage of the process and to transfer the material between different stages of the process. For washing and grading plants designed to produce one or more grades of aggregate and one or more grades of sand, the throughput of the plant (in terms of tonnes of material processed per hour) is typically limited by the sand washing and grading part of the plant, typically due to limitations imposed by the pumps, hydrocyclones and screens used to wash and grade the sand. Furthermore, when adapted for high throughput (typically requiring a high volume of water), the sand washing and grading process tends to operate with a high turbidity due to difficulties in removing fines at high sand flow rates.